THE ORIGINS OF ZYMOTIC DISEASES 171 



method." l The histories of dysentery, 2 cholera, 3 small-pox, 4 

 and other complaints are almost as old. But, while we have 

 evidence of the extreme antiquity of many diseases, we have 

 no clear account of the origin of any one disease. All the 

 world over men are still brought into contact with saprophytic 

 organisms ; the total number of men has increased ; their 

 means of intercommunication, and therefore of spreading 

 disease, has increased ; yet still we never receive authentic 

 accounts of a new disease. A new disease, in fact, appears 

 always to be a disease newly noted, not one newly evolved. 

 No sooner do we hear of an hitherto unknown malady than 

 we begin to hear of its prevalence in past times. It seems 

 probable, therefore, that the human race has not only under- 

 gone particular phases of evolution against particular diseases, 

 but that it has undergone a general evolution against all 

 bacterial infection. In that case saprophytic organisms have 

 a greater gulf to bridge to-day than formerly. No doubt, 

 under exceptional circumstances, the gulf is sometimes 

 bridged, as in the septic infection of wounds. But even so 

 the disease never spreads to normal healthy individuals. 

 For obvious reasons, unless capable of infecting healthy 

 individuals under normal conditions of life, a disease cannot 

 persist. 



289. It would appear, indeed, that saprophytic organisms 

 are able to assume, with comparative ease, parasitic habits 

 under special circumstances. 5 The main obstacle to the 

 evolution of a new disease lies, therefore, not in the assumption 

 of parasitic habits, but in the evolution of the means of 

 passing in sufficient numbers from one healthy individual to 

 another. Unless such individuals are very susceptible (i. e. 

 susceptible to very small doses of bacteria), this evolution is 

 difficult, the bacteria perishing before it is complete. Were 

 the passage from a saprophytic to a parasitic existence easy, 

 we would, considering the number of saprophytic forms with 

 which we are surrounded, hear of a new disease every day. 

 Indeed the persistence of the higher plants and animals 

 would be impossible. 



290. Confirmation of the above conjecture is afforded by 

 the fact that children are less resistant than adults to diseases 

 against which their race has undergone little or no evolution. 

 They represent a stage in the life-history of the race when 



Hirsch, vol. iii, p. 169. 2 Op. cit., p. 285. 



Op. cit., vol. 1, p. 432. * Op. cit., p. 123. 



6 Possibly typhus and relapsing fever are examples. But they appear 

 only under special circumstances, and disappear when these are removed. 



